I can't imagine Dad in a wheelchair. My mental image of him hasn't sufficiently recalibrated. But he uses one now, for the occasional outing he makes, wheeled down to the local pub for the odd pint, or taken on an afternoon stroll. He's too wobbly on his feet to manage on his own. And when I visit him, I see this frailty first-hand. He shuffles along the care home corridor, keels to one side, all at sea.

Yet despite the evidence, when I'm apart from him, I struggle to believe it. In my head, just out of sight, my Dad is as he always was. Tall. Strong. Always busy.

Such physical decline seems like an injustice too far, heaped upon the rest. Just when we'd conceded to the mental decay, there's this too.

For me, Dad will always be the figure striding ahead of me on the cliff path, Jack Russell at his heels.

We would often go on walks together, usually around the headland opposite our house – a route we referred to as "the rockets and back" because of a pair of cone-shaped markers on the clifftop that look like nothing else, shooting 12 ft from the gorse. One is pink, the other black and white, and when sailors out at sea align them, they mark the position of a buoy on a particularly hazardous rocky pinnacle called the Runnel Stone.

There was always a fixed routine. First, Dad would decide which of his pairs of binoculars he was going to take. He had several, on a sliding scale of preciousness, with the Leicas in pole position.

Once that decision had been made, we'd choose which hats we were going to wear, from the pile in the closet. Itchy bobble hats when it was drizzling, over-large Panamas when the sun shone.

Then we'd set off down the lane in front of the house, followed by a motley succession of small animals – including, at one point, the family cat, a particularly knowing Siamese.

We'd pause, to gather "sticks" from the tamarisk bushes that are everywhere in the cove where we lived. Dad would strip them of their leaves, so they were like giant, pliable rats' tails. I'd get one too, always smaller than his.

For some reason, Dad didn't like to walk without them. In those days it wasn't about needing one to lean on, rather more about some sense of what a countryman did, I think, and also as a useful pointing device.

And God, the pointing. Each walk was like a little lesson. He would quiz me on the names of various plants on the banks alongside the path. Pink campion. Wild garlic. Great orange drifts of montbretia.

The boats visible on the horizon were also picked out, with the binoculars deployed to help. Oil tankers, fishing vessels, the Scillonian ferry making her daily pilgrimage to the Scilly Isles – all needed to be categorised and commented upon.

On misty days such discussions would be punctuated by the spooky moan of the buoy on the Runnel Stone. And when it was clear, we'd pause at the top of the steep ascent to look at the view: granite cliffs, sparkling sea, perhaps some dolphins or a seal if we were lucky. Then we'd turn, on autopilot, steps tracing a route we'd done so many times before.

I struggle to comprehend that Dad will never make that walk again. Will never make any cliff walk ever again. But I know that when I follow that route, for as long as I live, I'll have him by my side. Just out of view.